Force-balance beam systems which are connected to primary measuring elements, commonly include a fluid pressure couple which produces a fluid pressure output within a range which is proportional to the position within a range of mechanical movement of the primary element. The linkage between the primary element and the fluid pressure couple may direct the forces which produce pulsations/oscillations in that member of the fluid pressure linked to the primary element.
The fluid pressure couple may be no more than a flapper connected to the primary element positioned over the discharge of a fluid pressure nozzle, the back pressures of the nozzle being representative of the positions of the primary element. The flapper may be represented as a fulcrumed beam and the nozzle may be replaced by a valve connected to the flapper. In all events, referring to a fluid pressure couple connected to a primary element, the essential structure is focused on the flapper-beam on which the forces are balanced about its fulcrum as it positions the fluid pressure valve to generate a range of fluid pressure outputs. It is common to arrange the system so that the fluid pressure outputs are over a range from 3 to 15 psi in response to the positions over the range of the primary element. The linkage between the primary element and the flapper-beam is commonly adjusted to determine what range of primary element position will produce the 3-15 psi output fluid pressures.
The adjustable linkage between the primary element and the flapper-beam is referred to as the proportional band adjustment. If the full movement of the primary element is linked to the flapper-beam to produce the 3-15 psi outputs, the proportional band is referred to as "high". If but a small range of primary element movement produces the 3-15 psi outputs, the proportional band is referred to as "low". The settings of the proportional band are designated in percentage. As an example, 100% of the primary element movement results in the full, 3-15 psi fluid pressure output. Thus, the proportional band is 100%. If 20% of the possible primary element movement results in the 3-15 psi output, the proportional band is designated as 20%.
There are settings, or adjustments, of the proportional band linkage which centralize forces on the flapper-beam element of the fluid pressure couple to cause it to oscillate. The natural frequency of the flapper-beam is reached by the proportional band adjustment and the result is a continuous, self-perpetuating pulsation, or vibration, or flutter, or oscillation of the flapper-beam which is, of course, unacceptable as representative of the primary element movement or position.
The general approach to eliminate the runaway flutter of the flapper-beam has been to provide friction. The force of friction applied to the beam will control the unstable pulsation, or vibration, or flutter, but at the cost of significant hysteresis. Obviously, it is desirable to control the unstable condition in these instruments without accepting hysteresis.